The present invention relates generally to superconducting quantum interference devices (SQUIDs) and more particularly to improved sensors with small effective pickup loop areas and good shielding which provide high spatial resolution when used with scanning SQUID microscopes.
A typical SQUID magnetometer circuit uses a SQUID with its input coil connected to a pickup coil by a twisted pair of superconducting wires. The SQUID itself is typically housed in a superconducting shield to screen it from ambient magnetic fields and to minimize the effects of fields associated with operating the SQUID on the field distribution to be measured. Planar thin-film SQUIDs of low self-inductance (.about.100 pH) with integrated spiral input coils can be used with wire-wound magnetometer pickup loops to give improved sensitivity. However, for certain applications, such as studying flux trapping phenomena in a superconducting groundplane, a sensitive magnetometer with an extremely small pickup loop is required. Since standard SQUID systems cannot be used for this, the miniature, very sensitive magnetometer of FIG. 1 having high-spatial-resolution is provided by an integrated, planar thin-film magnetometer fabricated on a single chip. FIG. 1 (from IBM Technical Disclosure Bulletin Vol. 27, No. 10A, March 1985) shows the magnetometer, in which a single (or double) turn pickup loop 10 is fabricated on chip 12. Pickup loop 10 has very low inductance. A groundplaned low noise DC SQUID 14 is fabricated with, for example, a three-turn spiral input coil 16. Groundplane 18 acts to shield SQUID 14 from stray magnetic fields. An inverted structure is used to maximize the separation between the SQUID loop 14 and groundplane 18 for better coupling. Input/Output (I/O) pads 20 are also provided on chip 12. A low inductance stripline connector 22 is used to connect input coil 16 and pickup coil 10. Alternatively, stripline connector 22 can be attached directly to the junction region of SQUID 14 such that stripline connector 22 and pickup coil 10 form an integral part of the SQUID's self inductance.
A simple single ring SQUID having a sub-.mu.m.sup.2 pick-up area with the entire SQUID being scanned in close proximity to the surface requires deep (&lt;0.5 .mu.m) sub-.mu.m minimum feature size for all fabrication levels of the SQUID. The arrangement of FIG. 1 has a small pick-up loop that is connected to the rest of the SQUID structure some distance away by a section of low inductance interconnect. The pick-up loop can be an integral part of the SQUID or part of an input circuit that is directly connected to the input coil of a SQUID on the same chip or part of an input circuit that is subsequently wire bonded to the input coil of a SQUID on a physically separate chip. In the latter case, the technology for fabricating the loop can be very different from that used to fabricate the SQUID, while in the former a sub-.mu.m.sup.2 pick-up structure can be formed with only one fabrication level with deep sub-.mu.m feature size. Minimizing the interconnect inductance will improve sensitivity, especially in the case of fully integrated devices. For these devices, which can be roughly 10 times more sensitive than the two-chip implementation, the detailed design of the pick-up loop structure and the transition to a low-inductance interconnect arrangement is very important.
One implementation of a sub-.mu.m pick-up loop is shown in FIGS. 2 and 3. This is a two chip arrangement 30,31 with superconducting wire bonds 32,33. Pick-up loop structure 34 requires only one level of superconducting metal, 100 nm of Nb in this case. The linewidth is 0.25 .mu.m, pick-up loop 34 is 1 .mu.m across, and leads 35,36 have a center-to-center spacing of 0.5 .mu.m. The design has the drawback that while pick-up loop 34 itself has a well-defined 1 .mu.m.sup.2 area, leads 35,36 have a significant effective pick-up area amounting to about 0.5 .mu.m.sup.2 per .mu.m of length. The spatial resolution is thus significantly degraded, especially for targets such as a line current source where the magnetic field falls off as 1/r. This structure also has a high lead inductance of about 1 pH/.mu.m.
A significant improvement can be made by adding a single groundplane above or below leads 35,36. This reduces both the effective pick-up area of leads 35,36 and their inductance. The wider this groundplane is in the vicinity of the pick-up loop, however, the more it can distort the field being measured.